Role of Brain Derived Neurotrophic Factor in the Supraoptic nucleus on Response to Salt Loading

Purpose: Previous studies have shown that Brain Derived Neurotrophic Factor (BDNF) contributes to ionic plasticity of vasopressin neurons from the supraoptic nucleus of the hypothalamus (SON) in response to 7 days of salt loading. This ionic plasticity is mediated by BDNF dependent phosphorylation of TrkB receptors and downregulation of KCC2, which alter chloride homeostasis resulting in impairment of baroreceptor inhibition of vasopressin release and an increase in mean arterial pressure that is partially vasopressin dependent. However, the specific source of BDNF is yet to be elucidated. In this study, we used adeno-associated viral vectors with shRNA to test the hypothesis that the salt loading produces BDNF release from the SON which in turn contributes to changes in chloride homeostasis and increased blood pressure. Methods: Adult male Sprague Dawley rats (250-300 g b w) were anesthetized with isoflurane and bilaterally injected in the SON (300 nl) with either an AAV vector containing shRNA against BDNF or a control construct with an mCherry reporter. The vectors were injected at a titer of 1.0 X 1013 GC/ml (Vector Biolabs, Malvern, PA). Two weeks after the stereotaxic injections, some rats from each group were instrumented with radio telemetry transmitters using isoflurane anesthesia for recording mean arterial pressure and heart rate. Rats were then housed in commercial metabolism cages and given water or 2% NaCl to drink for 7 days. Volume of fluid intake, urine excretion, food intake, and body weight were measured during the study along with radio telemetry recording for heart rate, mean arterial pressure and activity. Brain regions were harvested for measuring gene expression and protein content at the end of the 7 days. Seven days of 2% salt loading increased TrKB phosphorylation in the SON of rats injected with the control vector. Results: Virally mediated BDNF knockdown in the SON of salt loaded rats decreased TrkB phosphorylation. However, the increases in blood pressure produced by salt loading were not different between these two groups. Both salt loaded groups also had comparable decreases in body weight and increases in fluid intake. Conclusions: The results suggest that, while locally produced BDNF contributes to TrKB phosphorylation in the SON during salt loading, it is not necessary for the increase in blood pressure.